JP7459371B2 - Carbon nanotube dispersion liquid for lithium ion battery electrodes - Google Patents

Description

The present invention relates to a carbon nanotube dispersion for lithium ion battery electrodes.
This application claims priority based on Japanese Patent Application No. 2021-115519 filed in Japan on July 13, 2021, the contents of which are incorporated herein.

A lithium ion secondary battery is a type of secondary battery in which lithium ions in an electrolyte are responsible for electrical conduction. Lithium ion secondary batteries have excellent properties such as high energy density, excellent charge energy retention characteristics, and small so-called memory phenomenon that reduces apparent capacity. Therefore, lithium ion secondary batteries are used in a wide range of fields such as mobile phones, smartphones, personal computers, hybrid cars, and electric cars.
Here, a lithium ion secondary battery mainly includes a positive electrode plate, a negative electrode plate, and the like. The above-mentioned positive electrode plate and negative electrode plate have an electrode layer (electrode composite material layer) formed on the surface of an electrode core material (also referred to as a current collector). The electrode layer is manufactured by applying a dispersion in which an electrode active material is mixed with a carbon nanotube dispersion containing a conductive aid (carbon nanotubes, etc.), a binder, and a solvent onto the surface of an electrode core material, and drying the dispersion. be able to.
As mentioned above, the electrode layer is manufactured by applying a carbon nanotube dispersion for lithium ion battery electrodes containing an electrode active material to the surface of the electrode core material. is required to have low viscosity.
Under such circumstances, Patent Document 1 discloses an anionic surfactant (A), a nonionic surfactant (B), and an anionic surfactant (C) which is a compound different from the anionic surfactant (A). Disclosed is a multi-walled carbon nanotube aqueous dispersion characterized in that multi-walled carbon nanotubes are dispersed in an aqueous solution containing. Further, Patent Document 2 discloses an aqueous carbon nanotube dispersion comprising (a) a polysaccharide, (b) carbon nanotubes, and (c) a water-soluble compound having a perfluoroalkyl group.

However, in these inventions, the dispersibility or storage stability of carbon nanotubes may be poor, and if a large amount of dispersant is added, it will affect battery performance (internal resistance, capacity), so there is a limit to the amount of dispersant added. . Therefore, there is a need for a dispersant that can reduce the viscosity of a conductive paste with a small amount.

International Publication No. 2010/041750 JP2012-56788A

The problem to be solved by the present invention is to provide a carbon nanotube dispersion for lithium ion battery electrodes that has a viscosity that makes it easy to apply even when the amount of dispersion resin blended is relatively small.

Under such circumstances, the present inventors have conducted extensive research and found that the above problems can be solved by using a dispersion resin (A) containing a certain amount of a polar functional group-containing resin (a). The present invention is based on this novel finding.
Thus, the present invention provides the following:
[1] A carbon nanotube dispersion liquid for use in a lithium ion battery electrode, comprising a dispersion resin (A), carbon nanotubes (B), and water, wherein the dispersion resin (A) contains a polar functional group-containing resin (a).
[2] The carbon nanotube dispersion for a lithium ion battery electrode according to [1], characterized in that the polar functional group-containing resin (a) is an ionic polyvinyl alcohol resin (a1) and/or a carboxymethyl cellulose (a2).
[3] The carbon nanotube dispersion for a lithium ion battery electrode according to [2], characterized in that the ionic polyvinyl alcohol resin (a1) is a sulfonic acid-modified polyvinyl alcohol resin (a1-1) and/or a carboxylic acid-modified polyvinyl alcohol resin (a1-2).
[4] The carbon nanotube dispersion for a lithium ion battery electrode according to [3], characterized in that the ionic polyvinyl alcohol resin (a1) is a sulfonic acid-modified polyvinyl alcohol resin (a1-1), and the content ratio of a monomer unit having a sulfonic acid group in the sulfonic acid-modified polyvinyl alcohol resin (a1-1) is 0.1 to 30 mass% relative to the total mass of the monomer units constituting the ionic polyvinyl alcohol resin (a1).
[5] The carbon nanotube dispersion for a lithium ion battery electrode according to [2], characterized in that the saponification degree of the ionic polyvinyl alcohol resin (a1) is 50 to 100 mol %.
[6] The carbon nanotube dispersion for a lithium ion battery electrode according to [1], characterized in that the BET specific surface area of the carbon nanotubes (B) is 50 to 1800 m ² /g, and in a Raman spectrum of the carbon nanotubes ( ^B ), the G/D ratio is 5 to 200, where G is the maximum peak intensity in the range of 1560 to 1600 cm ^-1 and D is the maximum peak intensity in the range of 1310 to 1350 cm -1.
[7] The carbon nanotube dispersion for a lithium ion battery electrode according to [1], wherein in a Bode plot obtained by impedance measurement, in which reactance is plotted on the vertical axis and frequency is plotted on the horizontal axis, a minimum value of reactance exists in a frequency range of 170 to 600 kHz, and the minimum value of reactance existing in the frequency range of 170 to 600 kHz is 1.8 times or more the reactance value at a frequency of 1 kHz.
[8] A carbon nanotube dispersion liquid containing a dispersion resin (A), carbon nanotubes (B), and water,
The carbon nanotube dispersion liquid is characterized in that the dispersion resin (A) contains a polar functional group-containing resin (a),
A carbon nanotube dispersion for a lithium ion battery electrode, in which a Bode plot obtained by impedance measurement, in which reactance is plotted on the vertical axis and frequency is plotted on the horizontal axis, has a minimum value of the reactance in a frequency range of 170 to 600 kHz.
[9] The carbon nanotube dispersion for a lithium ion battery electrode according to [8], wherein in a Bode plot obtained by the impedance measurement, in which reactance is plotted on the vertical axis and frequency is plotted on the horizontal axis, a minimum value of reactance exists in a frequency range of 170 to 600 kHz, and the minimum value of reactance existing in the frequency range of 170 to 600 kHz is 1.8 times or more the reactance value at a frequency of 1 kHz.
[10] A carbon nanotube dispersion for a lithium ion battery electrode, comprising the carbon nanotube dispersion according to any one of [1] to [9], and further comprising an electrode active material.
[11] An electrode layer for a lithium ion battery formed from the carbon nanotube dispersion liquid according to [10].
[12] A lithium ion battery electrode comprising the lithium ion battery electrode layer according to [11] and a metal current collector.
[13] A lithium ion battery having the lithium ion battery electrode according to [12] as a positive electrode and/or a negative electrode.
[14] A lithium ion battery having the lithium ion battery electrode according to [12] as a positive electrode.

The dispersion resin (A) incorporated in the carbon nanotube dispersion liquid for lithium ion battery electrodes of the present invention can sufficiently reduce the viscosity of the paste with a relatively smaller amount than the pigment dispersion resins conventionally used in carbon nanotube dispersion liquids for lithium ion battery electrodes, and also has good storage stability.

FIG. 2 is an explanatory diagram showing the correlation between the degree of growth of the structure of carbon nanotubes (B) and a Bode plot. FIG. 2 is an explanatory diagram showing the correlation between the particle size distribution of aggregates of primary particles of carbon nanotubes (B) and the ratio of the minimum value (X) to the value (Y). FIG. 2 is a cross-sectional view showing an example of a lithium ion battery electrode having a lithium ion battery electrode layer. FIG. 1 is a cross-sectional view showing an example of a lithium ion battery.

EMBODIMENT OF THE INVENTION Hereinafter, the form for implementing this invention is demonstrated in detail.
It should be noted that the present invention is not limited to the following embodiments, but should be understood to include various modifications that may be implemented without departing from the gist of the present invention.
Furthermore, in this specification, "carbon nanotube dispersion for lithium ion battery electrodes" may be abbreviated as "carbon nanotube dispersion" and "carbon nanotubes" may be abbreviated as "CNT".

<Carbon nanotube dispersion for lithium-ion battery electrodes>
The present invention is a carbon nanotube dispersion liquid for use in a lithium ion battery electrode, which contains a dispersion resin (A), carbon nanotubes (B), and water, wherein the dispersion resin (A) contains a polar functional group-containing resin (a).
In addition, the "carbon nanotube dispersion" in the present application can also be called "carbon nanotube aqueous dispersion" since it necessarily contains water as a solvent.

<Dispersion Resin (A)>
The dispersing resin (A) that can be used in the present invention contains a polar functional group-containing resin (a).

<Polar Functional Group-Containing Resin (a)>
The polar functional group-containing resin (a) is a resin containing a highly polar functional group, and the polar functional group is an ionic and/or nonionic functional group.The ionic functional group may be, for example, an acidic functional group such as a sulfonic acid group, a carboxyl group, a sulfate group, a phosphonic acid group, a phosphoric acid group, a phosphinic acid group, or a mercapto group, or a basic functional group such as a primary, secondary, or tertiary amino group, an ammonium group, an imino group, or a nitrogen-containing heterocyclic group such as pyridine, pyrimidine, pyrazine, imidazole, or triazole.The nonionic functional group may be, for example, an amide group, a polyoxyalkylene group, or a pyrrolidone group.
Among them, as the polar functional group-containing resin (a), an ionic polar functional group-containing resin (a) is preferred, an ionic polyvinyl alcohol resin (a1) and/or a carboxymethyl cellulose (a2) is more preferred, and a carboxymethyl cellulose (a2) is even more preferred.

[Ionic polyvinyl alcohol resin (a1)]
The ionic polyvinyl alcohol resin (a1) is the aforementioned polyvinyl alcohol resin having an ionic functional group. Among these, it is preferable to have an acidic functional group as the ionic functional group, more preferably to have a sulfonic acid group or a carboxyl group, and particularly preferably to have a sulfonic acid group.
The acidic functional group may be in the form of a free acid, or an alkali metal salt such as a sodium salt or potassium salt, or an ammonium salt.
The ionic polyvinyl alcohol resin (a1) is preferably a sulfonic acid-modified polyvinyl alcohol resin (a1-1) and/or a carboxylic acid-modified polyvinyl alcohol resin (a1-2), and the sulfonic acid-modified polyvinyl alcohol resin (a1-1) is more preferable.

(Sulfonic acid modified polyvinyl alcohol resin (a1-1))
The sulfonic acid-modified polyvinyl alcohol resin (a1-1) can be produced, for example, by the following method.
(1) A method of copolymerizing a compound containing a sulfonic acid group and a polymerizable unsaturated group with a fatty acid vinyl ester such as vinyl acetate, and further saponifying the resulting polymer.
(2) A method of Michael addition of vinylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, etc. to polyvinyl alcohol.
(3) A method of heating polyvinyl alcohol with a sulfuric acid compound solution (sulfuric acid aqueous solution, sodium sulfite aqueous solution, etc.).
(4) A method of acetalizing polyvinyl alcohol with a sulfonic acid group-containing aldehyde compound.
(5) A method of polymerizing polyvinyl alcohol by allowing a compound having a functional group such as an alcohol having a sulfonic acid group, an aldehyde, and a thiol to coexist as a chain transfer agent.

Although any of the production methods can be suitably used, in particular, the polymer obtained by copolymerizing the compound containing a sulfonic acid group and a polymerizable unsaturated group (1) with a fatty acid vinyl ester such as vinyl acetate, Furthermore, a method of saponification is preferred.
As the compound containing a sulfonic acid group and a polymerizable unsaturated group, any compound that can be copolymerized with a fatty acid vinyl ester can be used without particular restriction.Specifically, examples include vinyl sulfonic acid, isoprene, Olefin sulfonic acids such as sulfonic acid, ethylene sulfonic acid, allyl sulfonic acid, and meta-allylsulfonic acid; sulfoalkyl maleates such as sodium sulfopropyl 2-ethylhexyl maleate, sodium sulfopropyl tridecyl maleate, and sodium sulfopropyleicosyl maleate; Sulfoalkyl (meth)acrylamides such as 2-(meth)acrylamido-2-methylpropanesulfonic acid and sodium N-sulfoisobutylene acrylamide; 3-methacryloyloxypropanesulfonic acid, 4-methacryloyloxybutanesulfonic acid, 3-methacryloyloxy- Sulfoalkyl (meth)acrylates such as 2-hydroxypropanesulfonic acid, 3-acryloyloxypropanesulfonic acid, sulfoethyl (meth)acrylate, sodium methacrylate-4-styrene sulfonate, sodium 2-sulfoethyl acrylate; allyloxybenzenesulfonic acid , methalyloxybenzenesulfonic acid, styrenesulfonic acid, oleyl 2-hydroxy-[3-allyloxy]-propylsulfosuccinate ammonium salt, etc., and these may be used alone or in combination of two or more. be able to.

The content of the polymerizable unsaturated monomer unit having a sulfonic acid group in the sulfonic acid-modified polyvinyl alcohol resin is preferably 0.1 to 30% by mass based on the total mass of the monomer units constituting the sulfonic acid-modified polyvinyl alcohol resin. , more preferably 0.2 to 10% by mass.

In the present invention, the "content ratio of monomer units having sulfonic acid groups in resin (a1-1)" refers to the content ratio of monomer units having sulfonic acid groups in the monomer mixture that is the raw material for resin (a1-1). means. Therefore, the content of the monomer having a sulfonic acid group in the resin (a1-1) is 0.1 to 30% by mass based on the total mass of monomer units constituting the resin (a1-1). means that the resin (a1-1) is a copolymer of raw material monomers containing 0.1 to 30% by mass of a monomer having a sulfonic acid group based on the total mass of the raw material monomers. Similarly, "the content of monomer units X in resin Y" means the content of monomer X in the monomer mixture that is the raw material for resin Y. Therefore, the content ratio of the polymerizable unsaturated group-containing monomer units X in the resin Y is a mass% with respect to the total mass of the monomer units constituting the resin Y. It means that it is a copolymer of raw material monomers containing a mass % of monomer X based on the total mass. In addition, when hydrolyzed by saponification, it is converted to the mass after saponification.

Examples of fatty acid vinyl esters that copolymerize with the above-mentioned compounds containing sulfonic acid groups and polymerizable unsaturated groups include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl caprate, vinyl caprylate, vinyl caproate, vinyl laurate, vinyl myristate, vinyl palmitate, vinyl stearate, vinyl pivalate, vinyl octylate, vinyl monochloroate, vinyl benzoate, vinyl cinnamate, vinyl crotonate, divinyl adipate, and derivatives thereof, which may be used alone or in combination of two or more. Of these, vinyl acetate is preferred.

Other copolymerizable polymerizable unsaturated monomers include, for example, olefinic monomers such as ethylene and propylene; Acryloyl group-containing monomers; allyl ethers such as allyl glycidyl ether; halogenated vinyl compounds such as vinyl chloride, vinylidene chloride, vinyl fluoride; and vinyl ethers such as alkyl vinyl ethers and 4-hydroxy vinyl ethers. These can be used alone or in combination of two or more.

The polymerization degree of the sulfonic acid-modified polyvinyl alcohol resin (a1-1) is preferably from 100 to 4,000, and more preferably from 100 to 3,000.
In this specification, the degree of polymerization can be calculated based on the molecular weight of the resin.
The molecular weight is a value obtained by converting the retention time (retention volume) measured using a gel permeation chromatograph (GPC) into the molecular weight of polystyrene using the retention time (retention volume) of a standard polystyrene with a known molecular weight measured under the same conditions. Specifically, the gel permeation chromatograph is "HLC8120GPC" (trade name, manufactured by Tosoh Corporation), and the four columns "TSKgel G-4000HXL", "TSKgel G-3000HXL", "TSKgel G-2500HXL" and "TSKgel G-2000HXL" (trade names, all manufactured by Tosoh Corporation) are used, and the measurement can be performed under the conditions of a mobile phase of tetrahydrofuran, a measurement temperature of 40°C, a flow rate of 1 mL/min, and a detector RI.

The degree of saponification of the sulfonic acid-modified polyvinyl alcohol resin (a1-1) is preferably within the range of 60 to 100 mol%, more preferably within the range of 70 to 100 mol%, and 80 to 100 mol%. It is more preferably within the range, and particularly preferably within the range of 90 to 99.9 mol%.
Note that in this specification, the degree of saponification can be measured by a measuring method based on JIS K6726-1994, or a measuring method based on the measuring method and modified according to the resin composition.
Generally, the lower the degree of saponification (low polarity), the worse the solubility in aqueous solvents. In addition, when the degree of saponification (high polarity) is high, although the solubility in aqueous solvents is good, the resin becomes more soluble in the aqueous solvent, so its adsorption to carbon nanotubes deteriorates, and a steric repulsion layer cannot be formed, resulting in dispersion. This results in poor dispersibility and storage stability. However, in the present invention, the reason why ionic polyvinyl alcohol resins (sulfonic acid-modified polyvinyl alcohol resins, carboxylic acid-modified polyvinyl alcohol resins, etc.) are effective in improving the dispersibility and storage stability of carbon nanotube dispersions is that While ensuring solubility in aqueous solvents through the main chain of the resin, adsorption to carbon nanotubes can be improved by introducing specific ionic functional groups into the side chains of the resin. It is considered that both solubility and adsorption to pigments were achieved.

The above resin (a1-1) can be produced by a polymerization method known per se, for example, a method of solution polymerization in an organic solvent, but is not limited thereto, and may be, for example, bulk polymerization, emulsion polymerization, suspension polymerization, etc. When carrying out solution polymerization, it may be continuous polymerization or batch polymerization, and the monomer may be charged all at once or in portions, or may be added continuously or intermittently.

The polymerization initiator used in solution polymerization is not particularly limited, but specifically, for example, azobisisobutyronitrile, azobis-2,4-dimethylpaleronitrile, azobis(4-methoxy-2 azo compounds such as acetyl peroxide, benzoyl peroxide, lauroyl peroxide, acetylcyclohexylsulfonyl peroxide, 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate, etc. Percarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, diethoxyethyl peroxydicarbonate; t-butyl peroxyneodecanate, cumyl peroxyneodecanate, t -Perester compounds such as butyl peroxyneodecanate; known radical polymerization initiators such as azobisdimethylvaleronitrile and azobismethoxyvaleronitrile can be used.

The polymerization reaction temperature is not particularly limited, but can usually be set within a range of about 30 to 200°C.

The conditions for saponification are not particularly limited, and saponification can be performed by any known method. For example, it can be carried out by hydrolyzing the ester moiety in the molecule in an alcohol solution such as methanol in the presence of an alkali catalyst or an acid catalyst.
As the alkali catalyst, for example, alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, sodium methylate, sodium ethylate, potassium methylate, alcoholates, etc. can be used. As the acid catalyst, for example, an aqueous solution of an inorganic acid such as hydrochloric acid or sulfuric acid, or an organic acid such as p-toluenesulfonic acid can be used, but it is preferable to use sodium hydroxide.
The temperature of the saponification reaction is not particularly limited, but is preferably in the range of 10 to 70°C, more preferably 30 to 40°C. The reaction time is not particularly limited, but it is preferably carried out within a range of 30 minutes to 3 hours.

The above resin (a1-1) can be made into a solid or a resin solution substituted with an arbitrary solvent by removing the solvent and/or replacing the solvent after completion of the synthesis.
The solvent may be removed by heating at normal pressure or under reduced pressure.
As a method of solvent replacement, a replacement solvent may be introduced at any stage before desolvation, during desolvation, or after desolvation.

(Carboxylic acid modified polyvinyl alcohol resin (a1-2))
The carboxylic acid-modified polyvinyl alcohol resin (a1-2) preferably contains a carboxyl group and has a degree of saponification of 60 to 100 mol%, more preferably in the range of 70 to 100 mol%. It is preferably within the range of 80 to 100 mol%, more preferably within the range of 90 to 99.9 mol%.

The degree of polymerization of the above carboxylic acid modified polyvinyl alcohol resin (a1-2) is preferably 100 to 4,000, and more preferably 100 to 3,000.

The above-mentioned carboxylic acid-modified polyvinyl alcohol resin (a1-2) is obtained by a polymerization method known per se, for example, by copolymerizing a compound containing a carboxyl group and a polymerizable unsaturated group with a fatty acid vinyl ester such as vinyl acetate, It can be obtained by a method of further saponifying the obtained polymer.
Furthermore, a carboxyl group can also be introduced (contained) by modifying the polyvinyl alcohol resin after synthesis.

The content ratio of the polymerizable unsaturated monomer unit having a carboxyl group is preferably 0.1 to 30% by mass, more preferably is 0.2 to 10% by mass, more preferably 0.2 to 5% by mass.

Regarding the method of synthesizing resin (a1-2), the method described for resin (a1-1) can be suitably used.

(Polyvinyl alcohol resin (a1-3) containing other ionic functional groups)
As the ionic polyvinyl alcohol resin (a1), in addition to the above-mentioned sulfonic acid-modified polyvinyl alcohol resin (a1-1) and carboxylic acid-modified polyvinyl alcohol resin (a1-2), ionic polyvinyl alcohol resins other than sulfonic acid groups and carboxyl groups are used. A polyvinyl alcohol resin (a1-3) containing a functional group can be used.
Regarding ionic functional groups other than sulfonic acid groups and carboxyl groups, the above-mentioned acidic functional groups and basic functional groups can be suitably used, and acidic functional groups are particularly preferred.

The resin (a1-3) preferably does not contain sulfonic acid groups or carboxyl groups and has a saponification degree of 60 to 100 mol%, more preferably 70 to 100 mol%. , more preferably within the range of 80 to 100 mol%, particularly preferably within the range of 90 to 99.9 mol%.

The degree of polymerization of the polyvinyl alcohol resin (a1-3) containing an ionic functional group other than a sulfonic acid group and a carboxyl group is preferably 100 to 4,000, more preferably 100 to 3,000. .

The polyvinyl alcohol resin (a1-3) containing the above-mentioned sulfonic acid group and ionic functional group other than carboxyl group can be obtained by a polymerization method known per se, for example, a method of copolymerizing a compound containing an ionic functional group other than a sulfonic acid group and a carboxyl group and a polymerizable unsaturated group with a fatty acid vinyl ester such as vinyl acetate, and further saponifying the resulting polymer.

The content ratio of polymerizable unsaturated monomer units having ionic functional groups other than sulfonic acid groups and carboxyl groups is 0.1 to 30% relative to the total mass of monomer units constituting the polyvinyl alcohol resin (a1-3). The amount is preferably 0.2 to 10% by weight, and even more preferably 0.2 to 5% by weight.

Regarding the method of synthesizing resin (a1-3), the method described for resin (a1-1) can be suitably used.

[Carboxymethylcellulose (a2)]
The carboxymethyl cellulose (a2) is a compound having a structure in which some or all of the hydroxyl groups in the glucose residues constituting cellulose are substituted with carboxymethyl ether groups, and may be in the form of a salt. Examples of carboxymethylcellulose salts include metal salts such as carboxymethylcellulose sodium salt.

The weight average molecular weight of the carboxymethyl cellulose is preferably 5,000 to 500,000, more preferably 10,000 to 100,000.
In this specification, the weight average molecular weight can be measured by the molecular weight measuring method using gel permeation chromatography (GPC) described in the above-mentioned "Method for measuring degree of polymerization".

The degree of etherification of the carboxymethyl cellulose is preferably 0.5 to 1.5, more preferably 0.6 to 1.2.
Note that in this specification, the degree of etherification is measured using an ashing measurement method. Specifically, 0.6 g of carboxymethyl cellulose is dried at 105° C. for 4 hours. After accurately weighing the mass of the dry matter, it is wrapped in filter paper and incinerated in a porcelain crucible. Transfer the ash to a 500 mL beaker, add 250 mL of water and 35 mL of 0.05 mol/L sulfuric acid aqueous solution, and boil for 30 minutes. After cooling, excess acid is back titrated with a 0.1 mol/L aqueous potassium hydroxide solution. Note that phenolphthalein is used as an indicator. Using the measurement results, the degree of etherification is calculated from the following formula.
Degree of etherification=162×A/(10000-80A)
In the formula, A=(af-bf1)/mass of dry material (g)
A: Amount (mL) of 0.05 mol/L sulfuric acid aqueous solution consumed by bound alkali in 1 g of sample
a: Amount of 0.05 mol/L sulfuric acid aqueous solution used (mL)
f: Titer of 0.05 mol/L sulfuric acid aqueous solution b: Titration amount (mL) of 0.1 mol/L potassium hydroxide aqueous solution
f1: titer of 0.1 mol/L potassium hydroxide aqueous solution

The method for producing the carboxymethyl cellulose is not particularly limited, and can be produced by any known production method. For example, it can be produced by reacting cellulose with an alkali and adding an etherification agent to the alkali-modified cellulose to perform an etherification reaction.

<Other resins>
The dispersion resin (A) may optionally contain resins other than the above-mentioned resin (a).
Examples include acrylic resins other than resin (a), polyester resins, epoxy resins, alkyd resins, urethane resins, silicone resins, polycarbonate resins, chlorine resins, fluorine resins, polyvinyl acetal resins, and composite resins thereof. . These resins can be used alone or in combination of two or more.
Among these, it is preferable to use at least one type of polyvinylidene fluoride (PVDF).
Moreover, these resins can be blended into the carbon nanotube dispersion liquid as a pigment dispersion resin or as an added resin after pigment dispersion.

<Carbon nanotube (B)>
As the carbon nanotubes (B), single-walled carbon nanotubes or multi-walled carbon nanotubes can be used alone or in combination. In particular, from the viewpoint of viscosity, conductivity, and cost, it is preferable to use single-walled carbon nanotubes.

The average outer diameter of the carbon nanotubes (B) is preferably 0.5 to 30 nm, more preferably 0.7 to 20 nm, and particularly preferably 1 to 10 nm.
In this specification, the average value of the outer diameter of carbon nanotubes (B) is determined by observing 100 arbitrarily extracted carbon nanotubes with a transmission electron microscope, measuring the outer diameter of each, and calculating the average value. This is the value obtained by calculating.

The average length of the carbon nanotubes (B) is preferably 1 to 100 μm, more preferably 5 to 80 μm, and particularly preferably 10 to 60 μm.
In addition, in this specification, the average value of the length of carbon nanotubes (B) is determined by observing 100 arbitrarily extracted carbon nanotubes with a transmission electron microscope, measuring the length of each, and calculating the average value. This is the value obtained by calculating.

The average particle diameter (D50) of aggregates of primary particles of carbon nanotubes (B) is preferably less than 7 μm, more preferably 5 μm or less, and particularly preferably less than 4 μm.
In this specification, the average particle diameter (D50) is measured by diluting and stirring a carbon nanotube dispersion liquid with water to a measurement concentration, and using a particle size distribution measuring device using a laser diffraction scattering method (Microtrac manufactured by Bell, Inc., This is a value calculated by measuring the volume-based particle size distribution using Microtrac MT3000 (product name).

The BET specific surface area of the carbon nanotubes (B) is preferably within the range of 50 to 1,800 m ² /g, more preferably within the range of 600 to 1,600 m ² /g from the viewpoint of viscosity and conductivity. Preferably, it is more preferably within the range of 800 to 1400 m ² /g.
In addition, in this specification, the BET specific surface area is a value obtained by a measurement method based on "JIS Z8830 Specific surface area measurement method of powder (solid) by gas adsorption".

In general, single-walled carbon nanotubes have a large specific surface area, and multi-walled carbon nanotubes have a small specific surface area, so single-walled carbon nanotubes with a large specific surface area are suitable for electrode applications.

In the Raman spectrum of the carbon nanotube (B), the G/D ratio, where G is the maximum peak intensity in the range of 1560 to 1600 cm ⁻¹ and D is the maximum peak intensity in the range of 1310 to 1350 cm ⁻¹ , is usually 5 to 200, preferably 20 to 180, more preferably 40 to 150, and even more preferably 70 to 130.
Here, if the G/D ratio is 5 or more, the crystallinity is high and the conductivity is excellent.
In this specification, the Raman spectrum of the carbon nanotube is a value obtained by placing the carbon nanotube in a Raman microscope (XploRA, manufactured by Horiba, Ltd.) and measuring with a laser wavelength of 532 nm. The G/D ratio is a value calculated as the G/D ratio of the carbon nanotube when the maximum peak intensity in the range of 1560 to 1600 cm ⁻¹ of the obtained peaks in the spectrum is G and the maximum peak intensity in the range of 1310 to 1350 cm ⁻¹ is D.

It is preferable that the carbon nanotube (B) is one type of carbon nanotube.

<Conductive pigment other than carbon nanotube (B)>
The carbon nanotube dispersion liquid of the present invention may contain conductive pigments other than carbon nanotubes. Specific examples include conductive carbon (B-2) such as acetylene black, Ketjen black, furnace black, thermal black, graphene, and graphite. These conductive carbons (B-2) can also be used in combination of two or more types.

The average primary particle diameter of the conductive carbon (B-2) is preferably within the range of 10 to 80 nm, more preferably within the range of 20 to 50 nm, in view of the relationship between viscosity and conductivity. .
Here, the above average primary particle diameter means that the pigment is observed with an electron microscope, the projected area of each of the 100 particles is determined, and the diameter is determined assuming a circle equal to that area. The average diameter of primary particles determined by simply averaging the diameters of In addition, when the pigment is in an agglomerated state, the calculation is performed using the primary particles that constitute the agglomerated particles.

The BET specific surface area of the conductive carbon (B-2) is preferably within the range of 1 to 500 m ² /g, and preferably within the range of 30 to 150 m ² /g, in view of the relationship between viscosity and conductivity. It is even more preferable.

The conductive carbon (B-2) is preferably basic from the viewpoint of pigment dispersibility, and specifically, the pH is preferably 7.5 or higher, and 8.0 to 12.0. More preferably, it is 8.5 to 11.0.
Note that in this specification, the pH of conductive carbon (B-2) can be measured according to ASTM D1512.

Further, from the viewpoint of conductivity, the conductive carbon (B-2) preferably has a state in which the primary particles form a chain structure, and the structure index is in the range of 1.5 to 4.0. It is more preferably within the range of 1.7 to 3.2, particularly preferably within the range of 1.7 to 3.2.
The structure itself can be observed relatively easily even in images taken with an electron microscope, but the structure index is a numerical value that quantifies the degree of structure. The structure index can generally be defined as the value obtained by dividing the DBP oil absorption (ml/100g) by the specific surface area (m ² /g). When the structure index is 1.5 or more, a structure is developed and sufficient conductivity is easily obtained. In addition, if the structure index is 4.0 or less, the particle size will be appropriate for the DBP oil absorption amount, making it easier to secure a conductive path, so sufficient conductivity can be exhibited, and the carbon nanotube It becomes easier to adjust the viscosity of the dispersion liquid to an appropriate level.

<Solvent>
The carbon nanotube dispersion of the present invention contains water as a solvent, but may also contain a solvent other than water (especially an aqueous solvent that dissolves in water).
Examples of the aqueous solvent include alcohols such as ethanol, propanol, butanol, methyl cellosolve, butyl cellosolve, propylene glycol monomethyl ether, and N-methyl-2-pyrrolidone. These solvents can be used alone or in combination with water.

<Other additives>
The carbon nanotube dispersion of the present invention may contain components other than the above components (A), (B), and water (sometimes referred to as other additives). Examples of other additives include neutralizing agents, pH adjusters, pigment dispersants, antifoaming agents, preservatives, rust preventives, plasticizers, binders, and the like.

The initial viscosity of the carbon nanotube dispersion is preferably less than 15 Pa·s, more preferably 13 Pa·s or less, and particularly preferably less than 12 Pa·s.
In this specification, the initial viscosity of the carbon nanotube dispersion is determined by measuring the carbon nanotube dispersion using a cone and plate viscometer (manufactured by HAAKE, trade name: Mars 2, diameter 35 mm, 2° inclined cone and plate). This is the value measured at a shear rate of 5.0 sec ^-1 . The carbon nanotube dispersion liquid used for the measurement was used within 60 minutes immediately after the carbon nanotube dispersion liquid was produced.

The volume resistivity of the lithium ion battery electrode layer formed from the carbon nanotube dispersion liquid is preferably less than 12 Ω·cm, more preferably less than 10 Ω·cm, and particularly preferably 9 Ω·cm or less.
In this specification, the volume resistivity of the lithium ion battery electrode layer is a value obtained by measuring the film thickness of the electrode layer, and then measuring the resistance value with a resistivity meter (manufactured by Mitsubishi Chemical Analytech Co., Ltd., product name "Loresta-GP MCP-T610") using an ASP probe (manufactured by Mitsubishi Chemical Analytech Co., Ltd., product name "MCP-TP03P"), and multiplying the obtained resistance value by a resistivity correction factor (RCF) of 4.532 and the film thickness to calculate the volume resistivity.

<<Electrodes for lithium ion batteries>>
The lithium ion battery electrode of the present invention can be obtained by applying the carbon nanotube dispersion of the present invention to a metal current collector to form a lithium ion battery electrode layer.
FIG. 3 is a cross-sectional view showing an example of a lithium ion battery electrode having a lithium ion battery electrode layer. In FIG. 3, a metal current collector 101 is coated with the carbon nanotube dispersion of the present invention to form a lithium ion battery electrode layer 102, thereby forming a lithium ion battery electrode 100.

≪Lithium ion battery≫
The lithium ion battery of the present invention has the lithium ion battery electrode layer of the present invention only on the positive electrode, only on the negative electrode, or on both the positive electrode and the negative electrode.
FIG. 4 is a cross-sectional view showing an example of a lithium ion battery. In FIG. 4, a metal current collector (negative electrode) 101b, a lithium ion battery electrode layer (negative electrode) 102b, a separator 103, a lithium ion battery electrode layer (positive electrode) 102a, and a metal current collector (positive electrode) 101a are shown in this order. They are stacked to form a lithium ion battery. The metal current collector (negative electrode) 101b and the lithium ion battery electrode layer (negative electrode) 102b constitute the lithium ion battery negative electrode 100b, and the metal current collector (positive electrode) 101a and the lithium ion battery electrode layer (negative electrode) The positive electrode) 102a constitutes a positive electrode 100a for a lithium ion battery. At least one of the lithium ion battery electrode layer (negative electrode) 102b and the lithium ion battery electrode layer (positive electrode) 102a may be formed from the carbon nanotube dispersion of the present invention, and the other may be formed from the carbon nanotube dispersion of the present invention. It does not have to be formed from a dispersion. Separator 103 holds an electrolyte (not shown).

<Method for producing carbon nanotube dispersion>
The solid content in the carbon nanotube dispersion liquid of the first embodiment of the present invention (also simply referred to as "carbon nanotube dispersion liquid") is usually preferably less than 50 mass % relative to the total mass of the carbon nanotube dispersion liquid, more preferably 10 mass % or less, and particularly preferably 5 mass % or less. Note that the carbon nanotube dispersion liquid of the first embodiment of the present invention does not contain an electrode active material, while the carbon nanotube dispersion liquid for lithium ion battery electrodes of the second embodiment of the present invention described below contains an electrode active material.
The total solid content of the dispersed resin (A) in the carbon nanotube dispersion solid content of the present invention is usually preferably 70 mass% or less, more preferably 60 mass% or less, and even more preferably 50 mass% or less, relative to the total mass of the carbon nanotube dispersion solid content, which is suitable in terms of viscosity during pigment dispersion, pigment dispersibility, dispersion stability, production efficiency, etc.
The content of the dispersed resin (A) in the carbon nanotube dispersion liquid of the present invention is usually, relative to the total mass of the carbon nanotube dispersion liquid, preferably 0.2 mass% or more and 10 mass% or less, more preferably 0.3 mass% or more and 5 mass% or less, and particularly preferably 0.5 mass% or more and 3 mass% or less.
The solid content of carbon nanotubes (B) in the carbon nanotube dispersion solid content of the present invention is usually, relative to the total mass of the carbon nanotube dispersion solid content, preferably 30 mass% or more and less than 85 mass%, more preferably 40 mass% or more and less than 80 mass%, and even more preferably 50 mass% or more and less than 75 mass%, which is suitable from the standpoint of battery performance.
The content of carbon nanotubes (B) in the carbon nanotube dispersion of the present invention is usually preferably 0.2 mass% or more and 5 mass% or less, more preferably 0.3 mass% or more and 3 mass% or less, and particularly preferably 0.5 mass% or more and 1.5 mass% or less, relative to the total mass of the carbon nanotube dispersion.

Further, the content of the solvent in the carbon nanotube dispersion of the present invention is usually preferably 50% by mass or more and less than 100% by mass, more preferably 70% by mass, based on the total mass of the carbon nanotube dispersion. From the viewpoint of drying efficiency and paste viscosity, the content is preferably 80% by mass or more and less than 98.5% by mass.
Note that the total content of each component contained in the carbon nanotube dispersion of the present invention does not exceed 100% by mass based on the total mass of the carbon nanotube dispersion.

The carbon nanotube dispersion of the present invention can be prepared by combining each of the above-mentioned components with, for example, Scandix, paint shaker, sand mill, ball mill, pebble mill, LMZ mill, DCP pearl mill, planetary ball mill, homogenizer, twin-screw kneader, thin film rotating type It can be prepared by uniformly mixing and dispersing using a conventionally known dispersing machine such as a high-speed mixer.

Specifically, the carbon nanotube dispersion liquid of the present invention is prepared by mixing a dispersion resin (A), carbon nanotubes (B), and water, and then using a dispersion machine (Clearmix CLM-2.2S manufactured by M Techniques). Dispersion was carried out at a speed of 14,000 rpm using a high-pressure homogenizer (Nano Veita, manufactured by Yoshida Kikai Co., Ltd.) for 12 passes at 60 MPa. After that, it is preferable to perform further dispersion under specific dispersion conditions (number of passes).
The number of passes is preferably greater than 5 and less than 70, more preferably greater than or equal to 7.5 and less than 50, particularly preferably greater than or equal to 8 and less than 45.
Note that the number of passes is a unit indicating the number of theoretical processes, and is obtained from the following calculation formula.
Processing time required for 1 pass (h) = paste liquid volume (L) ÷ processing speed of disperser (L/h)
Number of passes (theoretical number of processing) = Processing time (h) ÷ Processing time required for one pass (h)

The carbon nanotube dispersion liquid of the present invention can be mixed with an electrode active material and used to produce an electrode layer for a lithium ion battery electrode, as described below.

Furthermore, in the present invention, it has been found that information about the degree of dispersibility of a carbon nanotube dispersion can be obtained from the value of absorbance at a wavelength (268 nm). The absorbance (wavelength: 268 nm) of the carbon nanotube dispersion is preferably 1.3 or more, more preferably 1.3 to 2.0, particularly preferably 1.3 to 1.8. .
When the absorbance is 1.3 or more, the dispersibility is moderately advanced, and when it is 2.0 or less, it becomes easy to prevent excessive dispersion.
In this specification, the absorbance of the carbon nanotube dispersion is measured by diluting the carbon nanotube dispersion with deionized water to give a CNT concentration of 0.001% by mass, stirring until uniform, and then filling the sample into a cell. This is the value obtained by measuring the absorbance at a wavelength (268 nm) using U-1900 (trade name, spectrophotometer, manufactured by Hitachi High-Technologies Co., Ltd.).

<Impedance spectrum>
The carbon nanotube dispersion according to the first embodiment of the present invention is characterized in that in a Bode plot obtained by impedance measurement, in which reactance is plotted on the vertical axis and frequency is plotted on the horizontal axis, the reactance has a minimum value in the frequency range of 170 to 600 kHz. The reactance minimum value is preferably in the frequency range of 180 to 600 kHz, and more preferably in the frequency range of 200 to 400 kHz.
In the present invention, reactance means the imaginary part of complex impedance.
In this specification, the Bode plot uses a two-pole electrode with an electrode distance of 9 mm, in which a 0.3 mm thick copper plate with a gold-plated surface faces each other, and the size of the electrode is 100 ^mm2 . A 20 ml cylindrical container is filled with 15 ml of carbon nanotube dispersion liquid, and the electrode is inserted so that it is completely buried in the paste. Next, a sinusoidal AC voltage with a peak-to-peak voltage of 0.1 V is applied to the carbon nanotube dispersion liquid at 25 ° C. using an impedance analyzer (manufactured by Keysight, product name "4294A"), and the complex impedance and phase difference are measured at 500 points while sweeping the frequency between 100 Hz and 100 MHz. From the obtained data, the reactance is plotted on the vertical axis and the frequency on the horizontal axis.

The magnification (ratio) of the minimum value of reactance to the reactance at a frequency of 1 kHz, expressed by the following formula, is preferably greater than 1.5 and less than or equal to 5.0, and preferably greater than or equal to 1.6 and less than or equal to 4.0. More preferably, it is 1.7 or more and 3.5 or less.
Multiplication factor (ratio) of the minimum value of reactance to the reactance at a frequency of 1kHz = [minimum value of reactance] ÷ [reactance at a frequency of 1kHz]

In the Bode plot, there may be two or more minimum values of reactance. This tendency is particularly noticeable when two or more types of carbon nanotubes (B) are used together, when carbon nanotubes (B) and other conductive carbons are used together, or when the dispersion time of the carbon nanotube dispersion liquid is extremely short. be. Further, depending on the type of carbon nanotube (B) used, there may be two or more minimum values of reactance. When there are two or more reactance minimum values, it is sufficient that at least one of the minimum values exists in the frequency range of 170 to 600 kHz.
Carbon nanotubes (B) form a structure in the carbon nanotube dispersion, and the minimum value of reactance in the Bode plot indicates the degree of growth of the structure of carbon nanotubes (B), that is, the primary particles of carbon nanotubes (B). The movement depends on the size of the aggregate. For example, as shown in FIG. 1, the stronger the interaction between the primary particles 10 and the higher the degree of growth of the structure, that is, the larger the aggregate of the primary particles of carbon nanotubes (B), the lower the minimum value of reactance in the Bode plot. tends to move more easily to the low frequency region.

In addition, in FIG. 1, (a) is a Bode plot when the degree of structure growth is high, and in FIG. 1, (b) is a Bode plot when the degree of structure growth is moderate. 1, (c) is a Bode plot when the degree of growth of the structure is low.
In the Bode plot, if the minimum value of reactance exists in the frequency range of 170 to 600 kHz, the structure of carbon nanotubes (B) has grown appropriately, that is, the structure of carbon nanotubes (B) in the carbon nanotube dispersion liquid This means that the primary particles are dispersed while maintaining an appropriate structure. Therefore, when an electrode layer is formed using the carbon nanotube dispersion of the present invention, the primary particles of carbon nanotubes (B) are dispersed in the electrode layer while maintaining an appropriate structure. Therefore, a conductive path is efficiently formed, and an electrode with excellent conductivity can be formed. In addition, the viscosity of carbon nanotube dispersions also tends to decrease.
The Bode plot can be obtained, for example, by impedance measurement as shown below.

(Preparation of measurement cell)
Copper plates with a thickness of 0.3 mm whose surfaces are gold-plated are placed opposite each other, and bipolar electrodes with a distance of 9 mm between the electrodes are used. The size of the electrode is 100 ^mm2 . A 20 ml cylindrical container is filled with 15 ml of carbon nanotube dispersion and inserted so that the electrode is completely submerged in the paste.

(Impedance measurement)
A sinusoidal AC voltage with a peak-to-peak voltage of 0.1 V is applied to the carbon nanotube dispersion liquid at 25° C. using an impedance analyzer, and the complex impedance and phase difference are measured at 500 points while sweeping the frequency between 100 Hz and 100 MHz. From the obtained data, a Bode plot is created by plotting reactance on the vertical axis and frequency on the horizontal axis.

In the Bode plot, the minimum reactance value (X) in the frequency range of 170 to 600 kHz is preferably 1.8 times or more, more preferably 2.0 times or more, even more preferably 2.0 to 3.5 times, and particularly preferably 2.3 to 3.2 times, the reactance value (Y) at a frequency of 1 kHz.
As mentioned above, in the Bode plot, there may be two or more minimum reactance values. Even if there are multiple minimum values in the range of 170 to 600 kHz, the calculation (X/Y) should be performed using the minimum value in the range of 170 to 600 kHz.

The ratio of the minimum value (X) to the value (Y) is influenced by the particle size distribution of aggregates of primary particles of carbon nanotubes (B). For example, as shown in (a) in FIG. 2, the more uniform the particle diameters of the aggregates of primary particles of carbon nanotubes (B), that is, the more uniform they are, the more the minimum value (X ) tends to increase. On the other hand, as shown in FIG. 2(b), as the particle size of the aggregates 30 of the primary particles 10 of carbon nanotubes (B) becomes more nonuniform, the ratio of the minimum value (X) to the value (Y) increases. tends to become smaller.
If the minimum value (X) is 1.8 times or more the value (Y), it means that the particle size of the aggregate of primary particles of carbon nanotubes (B) in the carbon nanotube dispersion is uniform. . Therefore, when an electrode layer is formed using a carbon nanotube dispersion for a lithium ion battery electrode containing the electrode active material of the present invention, the primary particles of carbon nanotubes (B) are more uniformly dispersed in the electrode layer. Therefore, a conductive path can be formed more efficiently, and an electrode layer with better conductivity can be formed.

≪Carbon nanotube dispersion liquid for lithium ion battery electrodes≫
As a second aspect of the present invention, a carbon nanotube dispersion (simply referred to as "active material") for lithium ion battery electrodes is prepared by further blending an electrode active material (sometimes simply referred to as "active material") to be described later with the above carbon nanotube dispersion. (Also referred to as "carbon nanotube dispersion liquid for lithium ion battery electrodes.")
In addition, in the present invention, a "coating film obtained by applying a carbon nanotube dispersion" may be referred to as an "electrode layer".

<Electrode active material>
As the electrode active material, those known per se can be suitably used.
For example, as the positive electrode active material, for example, lithium nickelate (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), lithium cobalt oxide (LiCoO ₂ ), LiNi _1/3 Co _1/3 Mn _1/3 O ₂ Examples include lithium composite oxides such as. These electrode active materials can be used alone or in combination of two or more. Further, examples of the negative electrode active material include Li-based compounds, Sn-based compounds, Si-based compounds, graphite (natural graphite, artificial graphite), low crystalline carbon (hard carbon, soft carbon), and the like. These electrode active materials can be used alone or in combination of two or more.

The solid content in the carbon nanotube dispersion for lithium ion battery electrodes of the present invention is usually 70% by mass or more and less than 100% by mass based on the total mass of the carbon nanotube dispersion for lithium ion battery electrodes. It is preferably 80% by mass or more and less than 99% by mass, and particularly preferably 90% by mass or more and less than 98% by mass.
The solid content of the electrode active material in the solid content of the carbon nanotube dispersion for lithium ion battery electrodes of the present invention containing the electrode active material is usually the same as the total mass of the solid content of the carbon nanotube dispersion for lithium ion battery electrodes. 90% by mass or more and less than 100% by mass, preferably 95% by mass or more and less than 100% by mass, more preferably 98% by mass or more and less than 100% by mass, in terms of battery capacity, battery resistance, etc. It is preferable because
The content of the electrode active material in the carbon nanotube dispersion for lithium ion battery electrodes of the present invention is usually 30% by mass or more and less than 100% by mass based on the total mass of the carbon nanotube dispersion for lithium ion battery electrodes. It is preferably at least 40% by mass and less than 99% by mass, particularly preferably at least 50% by mass and less than 98% by mass.

≪Production method of carbon nanotube dispersion for lithium ion battery electrodes≫
The carbon nanotube dispersion liquid for lithium ion battery electrodes of the present invention is prepared by first preparing a carbon nanotube dispersion liquid according to the first aspect of the present invention containing the above-mentioned component (A), component (B), and water. It can be obtained by blending an electrode active material into a nanotube dispersion.
Further, the carbon nanotube dispersion for a lithium ion battery electrode of the present invention may be prepared by simultaneously mixing the aforementioned component (A), component (B), water, and electrode active material.

The total solid content of the dispersion resin (A) in the solid content of the carbon nanotube dispersion for lithium ion battery electrodes containing the electrode active material of the present invention is usually the solid content of the carbon nanotube dispersion for lithium ion battery electrodes. From the viewpoint of battery performance, paste viscosity, etc., the amount is preferably 0.001 to 20% by mass, preferably 0.005 to 10% by mass, based on the total mass of.

The solid content of carbon nanotubes (B) in the solid content of the carbon nanotube dispersion for lithium ion battery electrodes containing the electrode active material of the present invention is usually the total solid content of the carbon nanotube dispersion for lithium ion battery electrodes. From the viewpoint of battery performance, the amount is preferably 0.01 to 30% by mass, preferably 0.05 to 20% by mass, and more preferably 0.1 to 15% by mass.

Further, the content of the solvent in the carbon nanotube dispersion for lithium ion battery electrodes containing the electrode active material of the present invention is usually 0.1 to 60% based on the total mass of the carbon nanotube dispersion for lithium ion battery electrodes. From the viewpoint of electrode drying efficiency and paste viscosity, it is preferably 0.5 to 50 mass %, more preferably 1 to 45 mass %.

≪Production method for electrodes for lithium ion batteries≫
As mentioned above, the electrodes of lithium ion secondary batteries are made by applying a carbon nanotube dispersion for lithium ion battery electrodes containing an electrode active material to the surface of an electrode core material (metal current collector) and drying it. It can be manufactured in
Furthermore, the carbon nanotube dispersion for lithium ion battery electrodes of the present invention can also be used as a primer layer between an electrode core material and an electrode layer.
The method for applying the carbon nanotube dispersion for a lithium ion battery electrode containing an electrode active material can be performed by a method known per se using a die coater or the like. The coating amount of the carbon nanotube dispersion liquid for lithium ion battery electrodes containing an electrode active material is not particularly limited, but for example, the thickness of the electrode layer after drying is in the range of 0.001 to 0.5 mm (preferably 0.01 to 0.5 mm). 0.4 mm). The temperature of the drying step can be appropriately set within the range of, for example, 80 to 200°C, preferably 100 to 180°C. The time for the drying step can be set as appropriate within the range of, for example, 5 to 1200 seconds, preferably 5 to 120 seconds.

≪Lithium-ion battery manufacturing method≫
The lithium ion battery of the present invention is a laminate in which the electrode of the lithium ion secondary battery of the present invention is used only as a positive electrode, only as a negative electrode, or as both a positive electrode and a negative electrode, and a separator is laminated between the positive electrode and the negative electrode. can be manufactured by forming a laminate and subsequently injecting an electrolyte into the resulting laminate.

The present invention will be described in more detail with reference to Examples below, but the present invention is not limited to these specific embodiments.

Hereinafter, the present invention will be explained in more detail with reference to Production Examples, Examples, and Comparative Examples, but the present invention is not limited thereto. In each example, "part" indicates part by mass, and "%" indicates mass %.

<Production of Dispersion Resin>
Production Example 1: Production of sulfonic acid modified polyvinyl alcohol resin In a reaction vessel equipped with a thermometer, a reflux condenser, a nitrogen gas inlet tube and a stirrer, 92 parts by mass of vinyl acetate and 8.0 parts by mass of sodium allylsulfonate as polymerizable monomers, methanol as a solvent, and azobisisobutyronitrile as a polymerization initiator were used to carry out a copolymerization reaction at a temperature of about 60 degrees, and unreacted monomers were removed under reduced pressure to obtain a resin solution. Next, a methanol solution of sodium hydroxide was added to carry out a saponification reaction, and the mixture was thoroughly washed and then dried with a hot air dryer. Finally, a polyvinyl alcohol resin No. 1 (PVA1) was obtained, which has a polymerization degree of 300, a saponification degree of 90 mol%, and contains a sulfonic acid group-containing monomer unit (an ionic functional group-containing monomer unit).

Production Example 2: Production of sulfonic acid modified polyvinyl alcohol resin In a reaction vessel equipped with a thermometer, a reflux condenser, a nitrogen gas inlet tube and a stirrer, 92 parts by mass of vinyl acetate and 2.0 parts by mass of sodium allylsulfonate as polymerizable monomers, methanol as a solvent, and azobisisobutyronitrile as a polymerization initiator were used to carry out a copolymerization reaction at a temperature of about 60 degrees, and unreacted monomers were removed under reduced pressure to obtain a resin solution. Next, a methanol solution of sodium hydroxide was added to carry out a saponification reaction, and the mixture was thoroughly washed and then dried with a hot air dryer. Finally, a polyvinyl alcohol resin No. 2 (PVA2) was obtained, which has a polymerization degree of 300, a saponification degree of 90 mol%, and contains a sulfonic acid group-containing monomer unit (an ionic functional group-containing monomer unit).

Production Example 3 Production of carboxylic acid-modified polyvinyl alcohol resin In a reaction vessel equipped with a thermometer, a reflux condenser, a nitrogen gas introduction pipe, and a stirrer, 92 parts by mass of vinyl acetate and 2.0 parts by mass of acrylic acid were added as polymerizable monomers. After carrying out a copolymerization reaction at a temperature of about 60 degrees using methanol as a solvent and azobisisobutyronitrile as a polymerization initiator, unreacted monomers were removed under reduced pressure to obtain a resin solution. Next, a methanol solution of sodium hydroxide was added to perform a saponification reaction, and after thorough washing, it was dried in a hot air dryer. Finally, polyvinyl alcohol resin No. 1 with a degree of polymerization of 300, a degree of saponification of 90 mol%, and a carboxyl group-containing monomer unit (ionic functional group-containing monomer unit) was obtained. 3 (PVA3) was obtained.

Production Example 3 Production of polyvinyl alcohol resin containing no polar functional groups Vinyl acetate as a polymerizable monomer, methanol as a solvent, and polymerization initiator were placed in a reaction vessel equipped with a thermometer, a reflux condenser, a nitrogen gas introduction tube, and a stirrer. After carrying out a copolymerization reaction using azobisisobutyronitrile at a temperature of about 60 degrees, unreacted monomers were removed under reduced pressure to obtain a resin solution. Next, a methanol solution of sodium hydroxide was added to perform a saponification reaction, and after thorough washing, it was dried in a hot air dryer. Finally, a polyvinyl alcohol resin No. 1 containing no polar functional groups with a degree of polymerization of 500 and a degree of saponification of 88 mol% was obtained. 4 (PVA4) was obtained.

<<Manufacture of carbon nanotube dispersion>>
Examples 1 to 12, Comparative Examples 1 to 3
The types and amounts of dispersion resin (A), carbon nanotubes (B), and water listed in Table 1 were mixed, and the mixture was heated at 14,000 rpm using a dispersion machine (Clearmix CLM-2.2S manufactured by M Techniques). Dispersion was carried out at a speed of 100 to 100 μm until the whole was uniform and the dispersed particle size (D50) was 90 μm or less. Subsequently, after 12 passes of dispersion at 60 MPa using a high-pressure homogenizer (manufactured by Yoshida Kikai Co., Ltd., Nanoveita), further dispersion was performed under the dispersion conditions (number of passes) listed in Table 1 to obtain a carbon nanotube dispersion (X-1). ~(X-15) was obtained. The resin compounding amounts in Table 1 are solid content values.
Note that the number of passes is a unit indicating the number of theoretical processes, and is obtained from the following calculation formula.
Processing time required for one pass (h) = paste liquid volume (L) ÷ processing speed of disperser (L/h)
Number of passes (theoretical number of processing) = Processing time (h) ÷ Processing time required for one pass (h)

The symbols for each raw material in Table 1 above are as follows.

[Dispersion resin]
CMC1: carboxymethyl cellulose sodium salt, weight average molecular weight 56,000, degree of etherification 0.7
CMC2: carboxymethylcellulose sodium salt, weight average molecular weight 18000, degree of etherification 0.7
Methyl cellulose: weight average molecular weight 30,000, degree of etherification 1.5
[carbon nanotube]
SWCNT1: single-walled carbon nanotube, specific surface area 1100 m ² /g, outer diameter 1.56 nm, G/D ratio 100
SWCNT2: single-walled carbon nanotube, specific surface area 1070 m ² /g, outer diameter 1.3 nm, G/D ratio 87
MWCNT: multi-walled carbon nanotube, specific surface area 170 m ² /g, outer diameter 9 nm, G/D ratio 0.96
The G/D ratio of the carbon nanotubes, the impedance of the carbon nanotube dispersion, and the absorbance of the carbon nanotube dispersion were measured by the following methods.

[G/D ratio measurement of carbon nanotubes]
The Raman spectrum of the carbon nanotube was measured by installing the carbon nanotube in a Raman microscope (XploRA, manufactured by Horiba, Ltd.) using a laser wavelength of 532 nm. Among the obtained peaks, the maximum peak intensity in the range of 1560 to 1600 cm ^-1 in the spectrum is G, and the maximum peak intensity in the range of 1310 to 1350 cm ^-1 is D, and the ratio of G / D is carbon nanotube. The G/D ratio was set to

[Impedance measurement of carbon nanotube dispersion]
(Preparation of measurement cell)
Copper plates with a thickness of 0.3 mm whose surfaces were gold-plated were placed opposite each other, and bipolar electrodes with a distance between the electrodes of 9 mm were used. The size of the electrode was 100 ^mm2 . A 20 ml cylindrical container was filled with 15 ml of each carbon nanotube dispersion obtained in Examples and Comparative Examples, and the electrodes were inserted so as to be completely buried in the paste.
(impedance measurement)
A sinusoidal AC voltage with a peak-to-peak voltage of 0.1 V was applied to each of the carbon nanotube dispersions obtained in Examples and Comparative Examples at 25° C. using an impedance analyzer (manufactured by Keysight, trade name “4294A”). Then, the complex impedance and phase difference were measured at 500 points while sweeping the frequency between 100 Hz and 100 MHz. From the obtained data, a Bode plot was created by plotting reactance on the vertical axis and frequency on the horizontal axis.
Next, the minimum value of reactance (in Examples and Comparative Examples, all minimum values were at one point) was confirmed, and the magnification of the reactance at a frequency of 1 kHz was calculated using the following formula.
Multiplication factor with 1kHz = [Minimum value of reactance] ÷ [Reactance at frequency 1kHz]

[Absorbance measurement of carbon nanotube dispersion]
The carbon nanotube dispersion was diluted with deionized water to give a CNT concentration of 0.001% by mass, and stirred until uniform. Next, the sample was filled into a cell, and the absorbance at a wavelength (268 nm) was measured using U-1900 (trade name, spectrophotometer, manufactured by Hitachi High-Technologies Corporation).

<Evaluation test>
The carbon nanotube dispersion produced by the above-mentioned manufacturing method was subjected to an evaluation test by the following evaluation method. The results of the evaluation test are shown in Table 1. In the present invention, it is important that the performance is excellent in all items in the evaluation test, and if any one of them is rated as failing, the carbon nanotube dispersion is deemed to have failed.

[Average particle diameter (D50)]
The obtained carbon nanotube dispersion was diluted with water to a measured concentration, stirred, and measured on a volume basis using a particle size distribution measuring device using a laser diffraction scattering method (manufactured by Microtrac Bell, product name: Microtrac MT3000). Particle size distribution was measured and the average particle diameter (D50) was calculated. Evaluation was made according to the following criteria. Basically, the smaller the average particle diameter (D50) is, the more preferable it is, and A and B are considered to be passed, and C is considered to be rejected.
A (very good): The average particle diameter (D50) is less than 4 μm.
B (good): The average particle diameter (D50) is 4 μm or more and less than 7 μm.
C (poor): Average particle diameter (D50) is 7 μm or more.

[Initial viscosity (dispersibility)]
The initial viscosity of the obtained carbon nanotube dispersion was measured at a shear rate of 5.0 sec ^-1 using a cone and plate viscometer (manufactured by HAAKE, product name: Mars2, diameter 35 mm, cone and plate inclined at 2°) and evaluated according to the following criteria. The lower the viscosity at the same concentration, the better, with A and B being pass and C being fail. The carbon nanotube dispersion used for the measurement was one that had been produced within 60 minutes.
A (very good): The initial viscosity is less than 12 Pa·s.
B (Good): The initial viscosity is 12 Pa·s or more and less than 15 Pa·s.
C (poor): The initial viscosity is 15 Pa·s or more.

≪Production of carbon nanotube dispersion and electrode layer for lithium ion battery electrode containing electrode active material Examples Y1 to Y12, Comparative Examples Y1 to Y3 (CNT dispersion)
Examples Z1 to Z12, Comparative Examples Z1 to Z3 (electrode layer)
59.1 parts of electrode active material [lithium iron phosphate particles with an olivine structure represented by the composition formula _LiFePO4 (average particle size 1.3 μm, BET specific surface area 12.0 m ² /g)] and SBR (styrene butadiene rubber) , manufactured by JSR Corporation, trade name TRD102A, solid content 48.5 wt%) in a mixer, and then 10 parts of the carbon nanotube dispersions obtained in Examples 1 to 12 and Comparative Examples 1 to 3, and 50 parts of water to prepare carbon nanotube dispersions for lithium ion battery electrodes containing electrode active materials (Examples Y1 to Y12, Comparative Examples Y1 to Y3).
Next, it was applied onto an OHP film (PPC/laser PET film, manufactured by Fujifilm Business Innovation Co., Ltd.) with a thickness of 100 μm using an applicator, and dried at 80 degrees to form the electrode layer (positive electrode layer) of a lithium-ion battery. ) was obtained. (Examples Z1 to Z12, Comparative Examples Z1 to Z3)

[Volume resistivity]
After measuring the film thickness of the obtained electrode layers (Examples Z1 to Z12, Comparative Examples Z1 to Z3), the resistance was The resistance value was measured with a resistivity meter (manufactured by Mitsubishi Chemical Analytech Co., Ltd., product name "Loresta-GP MCP-T610"), and the resistance correction factor (RCF) of 4.532 and the coating film were added to the obtained resistance value. The volume resistivity was calculated by multiplying by the thickness. Volume resistivity was evaluated according to the following criteria. The lower the volume resistivity is, the more preferable it is, and A and B are passed, and C is rejected.
A (very good): Volume resistivity is less than 10 Ω·cm.
B (good): Volume resistivity is 10 Ω·cm or more and less than 12 Ω·cm.
C (poor): Volume resistivity is 12 Ω·cm or more.

According to the present invention, it is possible to provide a carbon nanotube dispersion for lithium ion battery electrodes that has a viscosity that makes it easy to apply even if the amount of dispersion resin blended is relatively small.

10 Primary particles of carbon nanotubes (B) 30 Aggregates of primary particles of carbon nanotubes (B) 100 Lithium ion battery electrode 100a Lithium ion battery positive electrode 100b Lithium ion battery negative electrode 101 Metal current collector 101a Metal current collector ( positive electrode)
101b Metal current collector (negative electrode)
102 Lithium ion battery electrode layer 102a Lithium ion battery electrode layer (positive electrode)
102b Lithium ion battery electrode layer (negative electrode)
103 Separator 200 Lithium ion battery

Claims (11)

A carbon nanotube dispersion liquid containing a dispersion resin (A), a carbon nanotube (B), and water, the dispersion resin (A) containing a polar functional group-containing resin (a),
The polar functional group-containing resin (a) contains an ionic polyvinyl alcohol resin (a1),
The ionic polyvinyl alcohol resin (a1) is a sulfonic acid-modified polyvinyl alcohol resin (a1-1), and the content ratio of monomer units having a sulfonic acid group in the sulfonic acid-modified polyvinyl alcohol resin (a1-1) is , 0.1 to 30% by mass based on the total mass of monomer units constituting the ionic polyvinyl alcohol resin (a1), and the degree of saponification of the sulfonic acid-modified polyvinyl alcohol resin (a1-1) is 90 to 90%. 99.9% carbon nanotube dispersion for lithium ion battery electrodes. The carbon nanotube dispersion for a lithium ion battery electrode according to claim 1, wherein the polar functional group-containing resin (a) contains carboxymethyl cellulose (a2). The carbon nanotube (B) has a BET specific surface area of 50 to 1800 m ² /g, and in the Raman spectrum of the carbon nanotube (B), the maximum peak intensity within the range of 1560 to 1600 cm ^-1 is G, 1310 to 1350 cm ^- The carbon nanotube dispersion for a lithium ion battery electrode according to claim 1, wherein the carbon nanotube dispersion for lithium ion battery electrodes has a G/D ratio of 5 to 200, where D is the maximum peak intensity within the range of ¹ . the carbon nanotubes (B) have a BET specific surface area of 50 to 1800 m ² /g, and in a Raman spectrum of the carbon nanotubes (B), a G/D ratio is 70 to 130, where G ^is the maximum peak intensity in the range of 1560 to 1600 cm ⁻¹ and D is the maximum peak intensity in the range of 1310 to 1350 cm −1;
2. The carbon nanotube dispersion for a lithium ion battery electrode according to claim 1, wherein in a Bode plot obtained by impedance measurement, in which reactance is plotted on the vertical axis and frequency is plotted on the horizontal axis, a minimum value of reactance exists in a frequency range of 170 to 600 kHz, and the minimum value of reactance existing in the frequency range of 170 to 600 kHz is 1.8 times or more the reactance value at a frequency of 1 kHz. The carbon nanotube dispersion for a lithium ion battery electrode according to claim 1, wherein the carbon nanotube (B) is a single-walled carbon nanotube. The carbon nanotube dispersion for a lithium ion battery electrode according to claim 5 , wherein the carbon nanotubes (B) have a BET specific surface area of 50 to 1800 m ² /g. A carbon nanotube dispersion for a lithium ion battery electrode, which is obtained by adding an electrode active material to the carbon nanotube dispersion according to any one of claims 1 to 6 . An electrode layer for a lithium ion battery formed from the carbon nanotube dispersion according to claim 7 . An electrode for a lithium ion battery, comprising the electrode layer for a lithium ion battery according to claim 8 and a metal current collector. A lithium ion battery comprising the lithium ion battery electrode according to claim 9 as a positive electrode and/or a negative electrode. A lithium ion battery comprising the lithium ion battery electrode according to claim 9 as a positive electrode.

Images